Control system for leaning vehicle

ABSTRACT

A method of operating a leaning vehicle is described. The method includes: determining a steering torque exerted on a steering assembly; determining a speed of travel of the vehicle; determining a leaning angle of a frame of the vehicle with respect to a reference angle; and exerting a leaning torque on the frame relative to a pivotable frame member about a pivot axis in a first direction using an actuator in response to a steering torque exerted on the steering assembly in a second direction opposite the first direction when a speed of travel of the vehicle is above a threshold speed, thereby causing the vehicle to turn in the first direction.

CROSS-REFERENCE

The present application is a division of U.S. patent application Ser.No. 12/501,025, filed Jul. 10, 2009, the entirety of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method of operating a leaningvehicle.

BACKGROUND

Two-wheeled leaning vehicles such as bicycles and motorcycles aretypically steered by the driver slightly shifting his upper body in thedesired direction of the turn while pushing on the handle on the insideof the turn, causing what is known as “countersteering”. The driver thusapplies a torque to the handlebar in the direction opposite the intendeddirection of the turn. Attempting to turn the handlebar to the right,for example, initially turns the front wheel to the right, which causesthe vehicle to initially turn gently to the right. This gentle turn tothe right generates a roll moment on the vehicle to the left. The rollmoment causes the motorcycle to lean to the left, which can be augmentedby the driver leaning to the left, and as a result the vehicle executesa left turn. Some motorcycle drivers are taught to “push” on thehandlebar in the direction of the desired turn, e.g. push on the lefthandle to turn left.

Some people are not aware of the fact that they countersteer whiledriving motorcycles. Countersteering is second nature to experiencedmotorcycle drivers, because they have countersteered leaning vehiclessince their childhood bicycles, usually without realizing it even then.In addition, it is not intuitive to consciously think about turning thehandlebar in one direction to turn the vehicle in the oppositedirection. However, an experienced driver who is used to countersteeringmotorcycles may subconsciously countersteer any vehicle that feels likea motorcycle, and may find it confusing to drive steer-in-direction(i.e. non-countersteering) assisted-leaning three-wheeled vehicles withhandlebars that typically steer like conventional cars.

Free-leaning three-wheeled vehicles may allow the driver tocountersteer, but they generally have to overcome more resistance thantwo-wheeled vehicles when leaning. A motorcycle can be leaned merely byovercoming the gyroscopic effect of the wheels and tilting the vehicleto one side, whereas a three-wheeled vehicle must typically overcome agreater gyroscopic effect due to the three wheels, as may also have topivot its frame relative to the suspension systems of the twolaterally-spaced wheels. As a result, free-leaning three-wheeledvehicles may have a higher resistance to leaning, and as a result may bemore difficult or less enjoyable to drive than motorcycles.

In addition, three-wheeled leaning vehicles may exhibit undesiredleaning behaviours under certain conditions. For example, a free-leaningvehicle will tip over at rest unless it is supported by a stand. Somevehicles address this problem by providing a locking mechanism toprevent tipping over at rest, which the driver must either manuallyactivate every time the vehicle has stopped or manually release when thevehicle starts moving. All of these conditions may diminish theenjoyment of the driver.

Therefore, there is a need for a leaning three-wheeled vehicle thatremedies at least some of the above deficiencies.

SUMMARY

It is an object of the present invention to ameliorate at least some ofthe inconveniences present in the prior art.

In on aspect, the present provides a method of operating a leaningvehicle. The vehicle comprises a frame having a front portion and a rearportion. A pivotable frame member is pivotally connected to the frameabout a generally horizontal pivot axis. The frame is pivotable relativeto the pivotable frame member between an upright position and aplurality of leaning positions. A steering assembly is supported by theframe for steering the vehicle. An actuator has a first portionconnected to the frame and a second portion connected to the pivotableframe member. The actuator is operative to selectively pivot the framewith respect to a reference angle about the pivot axis. The methodcomprises: determining a steering torque exerted on the steeringassembly; determining a speed of travel of the vehicle; determining aleaning angle of the frame with respect to the pivotable frame member;and exerting a leaning torque on the frame relative to the pivotableframe member about the pivot axis in a first direction using theactuator in response to a steering torque exerted on the steeringassembly in a second direction opposite the first direction when thespeed of travel is above a threshold speed, thereby causing the vehicleto turn in the first direction.

In a further aspect, the frame is prevented from pivoting about thepivot axis relative to the pivotable frame member when the speed oftravel is zero.

In a further aspect, a leaning torque is exerted in the directionopposite the leaning angle using the actuator when a reduction in themagnitude of at least one of the steering torque and the speed of travelis detected.

In an additional aspect, the method also comprises determining theleaning angle of the frame with respect to the reference angle comprisesdetermining the leaning angle of the frame with respect to a verticalorientation.

In a further aspect, determining the leaning angle of the frame withrespect to the reference angle comprises determining the leaning angleof the frame with respect to the pivotable frame member.

In an additional aspect, determining the leaning angle of the frame withrespect to the reference angle comprises determining a leaning angle ofthe frame with respect to a road surface.

In a further aspect, the threshold speed is a first threshold speed, andthe method further comprises exerting the leaning torque using theactuator in a direction opposite the leaning angle at least when thespeed of travel is below a second threshold speed, the second thresholdspeed being less than the first threshold speed.

In an additional aspect, when the speed of travel is below the secondthreshold speed, the leaning torque exerted by the actuator increases inmagnitude as the speed of travel decreases.

For purposes of this application, angle measures and angular quantitiessuch as torque are considered to be positive in a clockwise(right-turning) direction and negative in a counter-clockwise(left-turning) direction. For example, to turn the vehicle to the left,the driver initiates the turn by exerting a positive (countersteering)torque to turn the handlebar to the right, i.e. clockwise about thesteering column axis. As a result, the frame of the vehicle experiencesa negative torque to the left, resulting in a negative lean angle to theleft, i.e. counterclockwise as seen by the driver, and as a result thevehicle executes a left turn, i.e. counterclockwise as seen from above.

Embodiments of the present invention each have at least one of theabove-mentioned object and/or aspects, but do not necessarily have allof them. It should be understood that some aspects of the presentinvention that have resulted from attempting to attain theabove-mentioned object may not satisfy this objects and/or may satisfyother objects not specifically recited herein.

Additional and/or alternative features, aspects, and advantages ofembodiments of the present invention will become apparent from thefollowing description, the accompanying drawings, and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, as well as otheraspects and further features thereof, reference is made to the followingdescription which is to be used in conjunction with the accompanyingdrawings, where:

FIG. 1 is a front, right perspective view of a three-wheel leaningvehicle including a front suspension in accordance with an embodiment ofthe invention;

FIG. 2 is a front elevation view of the front suspension of thethree-wheel vehicle shown in FIG. 1 including a portion of the vehicleframe;

FIG. 3 is a front, left perspective view of a brake and a spindle of thethree-wheel shown in FIG. 1, to which the front suspension system isattached;

FIG. 4 is a top plan view of the front suspension and frame of thethree-wheel vehicle of FIG. 1;

FIG. 5 is a top, right perspective view of the front suspension andframe of the three-wheel vehicle of FIG. 1;

FIG. 6 is a rear left perspective view of the front suspension and frameof the three-wheel vehicle of FIG. 1;

FIG. 7 is a partially cut-away front left perspective view of the frontsuspension and frame of FIG. 5;

FIG. 8 is a front elevation view of the front suspension and frame ofFIG. 5, with the vehicle leaning to the left;

FIG. 9 is a block diagram of a control unit for controlling an operationof the vehicle and the components connected thereto according to anembodiment of the invention; and

FIGS. 10A-10E are graphical representations of control maps for thecontrol unit of FIG. 9 according to different embodiments of theinvention.

DETAILED DESCRIPTION

FIG. 1 illustrates a three-wheel leaning vehicle 10 in accordance withan embodiment of the invention. The particular aesthetic design detailsof the three-wheel vehicle 10 are not critical to this invention, andFIG. 1 merely illustrates one possible configuration. The three-wheelleaning vehicle 10 has a left side LS, a right side RS, a front F, and arear R as viewed by a driver driving the vehicle. The vehicle 10includes a frame 58 that supports and houses an engine 28, which couldbe any suitable type of power source such as an internal combustionengine or an electric motor. A straddle-type seat 16 is mounted on theframe 58 and preferably has a driver seat portion and a passenger seatportion 18 disposed behind the driver seat portion. The leaning vehicle10 features two front wheels 12; one on the left side and one on theright side of a longitudinal axis 146, and a single central rear wheel14. The central rear wheel 14 is suspended by a rear suspension system15 attached to the rear portion of the frame 58 and is operativelyconnected to the engine 28 through a suitable power transmissionmechanism such as a gearbox or a continuously-variable transmissioncoupled to an endless belt, chain, or driveshaft assembly. A steeringmechanism such as a handlebar 20 in front of the seat 16 is connected tothe front wheels 12 via a steering column 22 to steer the vehicle 10.Left and right suspension assemblies 24, 26 attach the front wheels 12to the vehicle 10 to permit the steering of each wheel 12 about asubstantially vertical steering/king pin axis 50 and tilting of eachwheel 12 about wheel tilting axis 52.

A pair of foot pegs 30 (only one of which is shown) project from vehicle10 so that the driver may rest his/her feet thereupon while driving. Abrake pedal 32 is situated in front of the right foot peg 30 to applythe front brakes 34 and rear brake (not shown). The vehicle 10 alsoincludes a plurality of fairings 35, 36, 38, and 40 which serve toprotect the vehicle components from the elements during use and renderthe vehicle aerodynamically efficient and aesthetically pleasing. Awindshield (not shown) may be placed in front of the handlebar 20 toprotect the driver from oncoming wind. Left and right passenger handles44 are attached to each side of the passenger seat portion 18. A muffler46, in fluid communication with the engine 28 via pipe 48, is attachedto the rear R of the vehicle 10. It is contemplated that any suitablealternative configuration of the pipe 48 may be used.

In operation, the left and right suspension assemblies 24 and 26 areconnected to the frame 58 of the vehicle 10, as described herein below,to permit the frame 58 and therefore the driver and the single centralrear wheel 14 to lean towards the right side or the left side much likea motorcycle. Additionally, the front wheels 12 are connected to theleft and right suspension assemblies 24 and 26 in such a way that thefront wheels 12 also tilt when the frame 58 is leaning into a cornerthereby duplicating a motorcycle behavior and driving style with thethree-wheel vehicle 10.

With reference to FIGS. 2-8, the suspension assemblies 24, 26 will bedescribed in detail. The suspension assemblies 24, 26 are mirror imagesof each other and function in substantially the same manner, and as suchsome components will be shown and/or described with respect to only oneor the other of the suspension assemblies 24, 26.

With reference to FIGS. 2 and 3, the front suspension assembly 26includes a lower suspension arm 54 pivotally attached at a first end 56to a bracket 57 rigidly attached to a lower portion of the frame 58,defining a first pivot point 60. The lower suspension arm 54 is alsopivotally attached at a second end 64 to a lower portion 78 of a spindle82, defining a second pivot point 66 as well as the wheel tilting axis52. The spindle 82 passes through a knuckle 81 and aligns with thesteering/king pin axis 50 such that the knuckle 81 may rotate relativeto the spindle 82 to steer the wheel 12 about steering/king pin axis 50.Bearings 83 (FIG. 3) provide smooth rotation between the spindle 82 andthe knuckle 81. A hub or bearing 84 is used to attach the front wheel 12to the front suspension assembly 26 such that the front wheel 12 rotatesabout rotation axis 142. The knuckle 81 also includes a steering arm 86.A leaning rod 90 connects the top portion 88 of the spindle 82 to theframe 58. A steering rod 130 connects the steering arm 86 of knuckle 81to a steering mechanism 126 (as best seen in FIGS. 5 and 6).

With reference to FIG. 4, the lower suspension arm 54 includes a frontportion 68 and a rear portion 70 to form a ‘Y’ or ‘V’ shape. The lowersuspension arm 54 is attached to the frame 58 at a front location 72(FIG. 2) and a rear location 74 (FIG. 5) which together define thepivoting axis 76 of the lower suspension arm 54. The pivoting axis 76passes through pivot point 60. The lower suspension arm 54 also includesan upwardly curved portion 144, best seen in FIG. 2, between first andsecond ends 56 and 64. The upwardly curved portion 144 allows forclearance between the wheel 12 and the suspension arm 54 when thevehicle 10 is leaning during a turn. It is to be understood that theupwardly curved portion 144 lies above a plane including the pivotingaxis 76 and the wheel tilting axis 52.

With reference to FIG. 5, the frame 58 includes a lower member 59, anupright member 55 and an upper member 51. The lower member 59 is curvedupwardly at the rear (not shown) to connect with the upper member 51.The upright member 55 joins the front of the upper member 51 to thefront of the lower member 59 to form a rigid frame 58. Brackets 57 and53 are connected to the lower member 59 at forward location 72 and rearlocation 74 respectively to which the front and rear portions 68 and 70of the lower suspension arm 54 are connected. An upper end 150 of thesteering column 22 passes through an aperture 152 in the upper member 51of the frame 58 and is supported by a bearing (not shown) mounted to theupper member 51 or by other components of the vehicle 10. The steeringcolumn 22 is connected to a steering linkage 126 which in turn isconnected to a proximal end 128 of the steering rod 130. A distal end132 of the steering rod 130 is connected to the steering arm 86 of theknuckle 81. Preferably, the proximal end 128 and the distal end 132 ofthe steering rod 130 are connected to the steering linkage 126 andsteering arm 86 via ball joints 134 to allow the spindle 82 tosimultaneously lean and steer. The steering rod 130 and the ball joint134 are pivotally connected to the steering linkage 126 via a pivotconnection which allows rotation about the axis 136. The combination ofthe ball joint 134 and the pivot connection about the axis 136 allowsfor large leaning angles and large steering angles without binding ofthe steering components.

FIG. 5 illustrates the steering/king pin axis 50 which is defined by therotation of the spindle 82 relative to the knuckle 81 to steer thevehicle 10. FIG. 5 also illustrates the wheel tilting axis 52 defined bythe connection of the lower portion 78 with the end 64 of the lowersuspension arm 54 about which the wheel 12 may tilt towards the frame 58or away from the frame 58.

Referring now to FIG. 6, a first end 92 of the leaning rod 90 isconnected to the top portion 88 of spindle 82 (FIG. 2) and a second end94 of the leaning rod 90 is connected to the upright member 55 of theframe 58. The upright member 55 of the frame 58 is therefore directlyconnected to the top portion 88 of the spindle 82 to push or pull thespindle 82, via the leaning rod 90, to pivot about the wheel tiltingaxis 52 when the frame 58 is leaning. The wheel 12 is therefore tiltedwhen the frame 58 is leaning to one side or the other. Preferably, balljoints are used to connect the leaning rod 90 to the top portion 88 ofthe spindle 82 and the frame 58 so that the leaning rod 90 may only besubjected to tension and compression forces when pushing or pulling thespindle 82. The leaning rod 90 provides a rigid link to maintainalignment between the leaning angle of the frame 58 and the wheelcamber, and as a result the gyroscopic stability of the wheel camberprovides stability to the orientation of the frame 58 at high speeds.

With reference to FIGS. 2, 6 and 7, a pivotable frame member in the formof a shock tower 96 is pivotally attached at a lower end 98 to the frame58, such that the frame 58 pivots with respect to the shock tower 96about frame pivot axis 100. The frame 58 and the shock tower 96 maytherefore pivot relative to one another about the pivot axis 100. Theupper end 102 of the shock tower 96 includes a bracket 104 having a leftside 106 and a right side 108. It is contemplated that the bracket 104may be formed integrally with the shock tower 96 in a one-piececonstruction. An electric actuator 112 is mounted on a forward portionof the frame 58, and selectively exerts a torque on the shock tower 96via an output gear 117 and a gear 118. The gear 118 is fixed to theshock tower 96 and is concentric with the frame pivot axis 100. It iscontemplated that the actuator 112 may be oriented vertically instead ofhorizontally, in which case the gears 117, 118 would be replaced bybevel gears or any other suitable gearing arrangement. It is furthercontemplated that any other suitable actuator may alternatively be used,such as a mechanical or hydraulic actuator. It is further contemplatedthat more than one actuator 112 may be provided. An upper end 114 of ashock absorber assembly 116 is attached to an extremity of the rightside 108 of the bracket 104 at a pivot point 115 while the lower end 122of the shock absorber assembly 116 is attached to the lower suspensionarm 54 at pivot point 123. A second shock absorber and actuator areconnected to the shock tower 96 and frame 58 on the left side of theshock tower 96, symmetrical about the frame pivot axis 100.

Referring back to FIG. 5, a brake disk 138 is fixed to rotate about theaxis 142. A brake caliper 140 is fixed to be stationary with the knuckle81. When the caliper 140 applies a braking force to the disc brake 138to reduce the rotational speed of the hub/bearing 84 and thus the wheel12, the spindle 82 is subjected to a torque in the direction of thearrow T (for a forward travelling direction of the vehicle). Because ofthe orientation of the axis 50, 52 and because the spindle 82 cannotrotate in the direction of the torque T relative to the lower suspensionarm 54, all of the torque T is transferred to the lower suspension arm54.

Having all the braking forces pass through the lower suspension arm 54permits the leaning rod 90 to have a small diameter and occupy verylittle longitudinal space when compared to an upper A-arm of aconventional double A-arm suspension designed to withstand brakingforces such as lower suspension arm 54. This leaves ample space for thewheel to tilt inwards without contacting other components, particularlywhen simultaneously steering the wheel through large steering angles.This design also allows for the necessary space to easily mount theshock absorber 116 to the shock tower 96.

Prior art designs having two A-arms, one situated above the other, allowthe arms to be smaller since the torsion forces are distributed betweenthe upper and lower A-arms. However this configuration can limit thedegree of leaning of the wheel. The single lower suspension arm 54 ofthe present invention is bulkier than typical double A-arms systems butallows the leaning rod 90 to be a small single rod thereby freeing spaceand allowing the wheels to lean farther than double A-arms systems.

The present configuration allows for sufficient space for all the frontsuspension components to articulate, lean, tilt and turn withoutinterfering with one another or with the actuator 112. As illustrated inFIG. 5, the shock absorber 116 lies in a substantially vertical plane141 substantially perpendicular to the frame pivot axis 100. Leaning rod90 and steering rod 130 also lie within substantially vertical planes135, 137 which are also substantially parallel to the plane 141 in whichthe shock absorber 116 lies.

It should be understood that while the frame 58 is leaning to the leftor right, the wheels 12 are also leaning to the left or right and couldalso be simultaneously steered. The leaning of the frame 58 and thewheels 12 lowers the steering and leaning rods 130, 90 toward the lowersuspension arm 54. Keeping the components in their respectivesubstantially vertical planes throughout the leaning process ensures nointerference between each component. Although simultaneously steeringthe wheel 12 about axis 50 while leaning the wheel 12 about axis 52 willcause some longitudinal movement of the steering and leaning rods 130,90, the longitudinal distance between the components combined with thecomponents remaining in substantially vertical planes ensures that thereis no interference between the components.

With reference to FIG. 8, in operation, the vehicle 10 is countersteeredsimilarly to a two-wheeled motorcycle in order to turn. Taking theexample of a left turn, the driver exerts a torque on the handlebar 20in the clockwise direction as seen from above, which rotates the knuckle82 clockwise about the axis 50. The gyroscopic forces acting on thewheel 12 exert a counterclockwise torque (as seen from the rear) aboutthe axis 52. This torque is transmitted to the frame via the knuckle 82and the leaning rod 90, causing the frame 58 to pivot counterclockwisewith respect to the shock tower 96 about the frame pivot axis 100, withthe result that the vehicle turns to the left. In the embodiment shown,the steering axis 50 of the inside wheel 12 remains substantiallyparallel to the frame 58 during a turn, whereas the steering axis 50 ofthe outside wheel 12 acquires a slight negative camber (not shown),however it should be understood that the respective steering axes 50 maytilt by different angles or in different directions depending on theparticular geometry of the vehicle 10.

As can be seen in FIG. 8, when the vehicle 10 is leaning into a turn,the shock tower 96 remains substantially upright while the frame 58pivots with respect to the shock tower 96 about the frame pivot axis 100such that the shock absorber assembly 116 of the front suspension is notdirectly involved in the leaning motion of the frame 58. The operationof the shock absorber assembly 116 is independent of the leaning motionof the frame 58. The motion ratio between wheels 12 and the shockabsorber assemblies 116 remains substantially constant while the frame58 is leaning to provide generally unaltered wheel dampening whileleaning into a turn and travelling over rough terrain at the same time.The motion ratio is the ratio between the vertical movement of the wheel12 and the stroke of the corresponding shock absorber 116. A personskilled in the art would recognise that a substantial change in motionratio due to the leaning of the frame 58 is not desirable. As can beseen in FIG. 8, the top pivot point 115 of shock absorber assembly 116remains at a constant distance d1 from the frame pivot axis 100 when theframe 58 is leaning. However, the bottom pivot point 123 of shockabsorber 116 (which is located on the lower suspension arm 54), followsthe marginal displacement of lower suspension arm 54 downward and inwardcaused by the rotational displacement of its first pivot point 60 aboutthe frame pivot axis 100 when the frame 58 is leaning to the right. Thedistance d3 defines the radius of the rotational displacement of thepivot point 60 about the frame pivot axis 100 when the frame 58 isleaning. By keeping d3 within a certain range, the change in motionratio is minimal. It is to be understood that by decreasing distance d3,the motion ratio becomes less affected by the leaning of the frame 58. Afully constant motion ratio can be obtained by having lower suspensionarm axis 76 (pivot point 60) coaxial with the frame pivot axis 100.

With reference to FIG. 9, a control unit 200 for controlling theactuator 112 and the components connected thereto will be described. Thecontrol unit 200 is electrically connected to a number of sensors,including a vehicle speed sensor 202 for detecting a speed of travel ofthe vehicle 10, a lateral acceleration sensor 204 for detecting alateral acceleration of the vehicle 10, a steering torque sensor 206 fordetecting a torque exerted on the steering mechanism, and at least onelean angle sensor 208 for detecting an angle of the frame 58 withrespect to a reference angle. The reference angle may be any suitableangle, such as a vertical orientation determined by gravity, the angleof the road surface, or the orientation of the shock tower 96. It iscontemplated that different reference angles may be used at differentvehicle speeds. For example, the reference angle may be a verticalorientation at low speeds and the orientation of the shock tower 96 athigh speeds. The control unit 200 receives signals from the sensors 202,204, 206, 208 regarding the operating state of the vehicle 10. It iscontemplated that some of these sensors may be omitted, or replaced withother sensors that provide similar information to the control unit 200.It is further contemplated that additional sensors of different typesmay be electrically connected to the control unit 200. The control unit200 is electrically connected to the actuator 112, and controls theoperation of the actuator 112 based on the signals received from thesensors.

With reference to FIGS. 10A-10D, the operation of the actuator 112 willbe described according to a number of alternative embodiments. Thecontrol unit 200 selectively causes the actuator 112 to exert a torquebetween the frame 58 and the shock tower 96 about the frame pivot axis100. The magnitude, direction, and duration of the torque generated bythe actuator 112 is determined by the control unit 200 based on theinputs received from one or more of the sensors and one or more storedcontrol maps. In the present embodiment, the torque depends on the speedof the vehicle 10 as indicated by the speed sensor 202, the torqueexerted on the steering column 22 as indicated by the steering torquesensor 206, and the current lean angle as indicated by the lean anglesensor 208. The torque T generated by the actuator 112 is calculated as

T=K ₀ ·θ+K _(s) ·S

where θ is the current lean angle with respect to the reference angle, Sis the difference between the current steering torque S_(T) on thesteering assembly and the steering torque S_(F) required to maintain theframe at the desired lean angle, and K_(θ) and K_(s) are quantitiesdetermined from the control map based on the signals received by thecontrol unit 200. It is contemplated that the term θ may be the currentlean angle with respect to any suitable reference angle such as thosedescribed herein. It is further contemplated that the term K_(θ)·θ mayalternatively be replaced by any suitable function of θ that producesthe desired performance characteristics. It is further contemplated thatthe term K_(θ)·θ may alternatively be replaced by a multivariatefunction of the current lean angles with respect to two or more suitablereference angles, for example a first component based on the lean anglewith respect to the shock tower 96 and a second component based on thelean angle with respect to vertical. The particular dependency on thelean angle θ may take any suitable form, provided the resulting torqueexerted on the frame 58 is a restoring torque urging the frame 58 towardthe reference angle. S is calculated as

S=S _(T) −S _(F)

where S_(F) is a function of the desired lean angle as shown in FIG.10E. The desired lean angle is determined based on a number ofparameters including vehicle speed, steering torque, and lateralacceleration. It should be understood that the effect of the K_(s)·Sterm is to generate a torque T on the frame until the terms S_(T) andS_(F) are equal in magnitude, which occurs when the lean angle θcorresponds to the steering torque S_(T).

Referring to FIG. 10A, according to a first embodiment K_(θ) is zerowhen the vehicle is travelling at a high speed, begins to increase inmagnitude when the vehicle speed is below a threshold speed v₂, andbecomes larger in magnitude as the vehicle speed approaches zero. Thesign of K_(θ) is always negative, indicating that the torque T resultingfrom the lean angle is a restoring force in the direction opposite thelean angle θ. The torque T opposes the frame 58 leaning with respect tothe reference angle, resulting in a “stiffness” or lean resistance thatincreases in magnitude as the vehicle slows, effectively limiting thelean angle to smaller values at slower speeds. When the vehicle isstopped, the actuator 112 may optionally lock the frame 58 in a verticalposition with respect to the reference angle. As a result, large leaningangles are prevented at low speeds, when excessive leaning could causediscomfort or inconvenience to the rider. In particular, when thevehicle 10 is stopped, the frame 58 is maintained in an upright positionso that the driver may maintain his seating position with his feet onthe foot pegs 30 instead of having to place one or both feet on theground to maintain the vehicle 10 upright as is commonly required on atwo-wheeled motorcycle. In addition, if the driver is attempting to parkthe vehicle 10 at low speeds he may exert significant torque on thehandlebar 20 to steer the vehicle 10, but may not desire the vehicle 10to lean significantly. In this embodiment, K_(s) is zero, such that thetorque T only has a lean angle component, and the vehicle 10 iseffectively free-leaning above the threshold speed v₂, with no leaningtorque being generated by the actuator 112. The vehicle 10 steers in amanner similar to a motorcycle, by countersteering and leaning the frame58 into the turn, with the leaning torque being generated by thedriver's countersteering and the geometry of the vehicle 10,independently of the actuator 112.

Referring to FIG. 10B, according to a second embodiment K_(θ) is similarto the embodiment of FIG. 10A, and provides a restoring torque to limitor prevent leaning at low speeds. In this embodiment, K_(s) is zero atlow speeds, such that the leaning behaviour of the vehicle 10 at lowspeeds is similar to the embodiment of FIG. 10A. At higher speeds, K_(s)gradually increases in magnitude until it reaches an approximatelyconstant value above a second threshold speed v₁. In this embodiment,when the speed is above v₁ and the driver exerts a steering torque s onthe handlebar 20 indicative of his desire to turn the vehicle 10, theactuator 112 assists the driver by applying a leaning torque T tosupplement the leaning torque resulting from the geometry of thevehicle. In this manner, the driver can effectively lean the vehicle 10during a turn, with less effort than would ordinarily be required in afree-leaning vehicle, at least at speeds above v₁. The sign of K_(s) isalways negative, indicating that the vehicle 10 is countersteered, i.e.the direction of the leaning torque T (and therefore the direction ofthe turn) is opposite the direction of the steering torque s.

Referring to FIG. 10C, according to a third embodiment K_(θ) is similarto the embodiment of FIG. 10A, and provides a restoring torque to limitor prevent leaning at low speeds. In this embodiment, K_(s) is zero atlow speeds, such that the leaning behaviour of the vehicle 10 at lowspeeds is similar to the embodiment of FIG. 10A. At higher speeds, K_(s)has a small negative value, such that when the driver exerts a steeringtorque s on the handlebar 20 the actuator 112 exerts a small leaningtorque T that is preferably approximately sufficient to overcome theinternal resistance of the actuator 112 and the front suspensionassemblies 24, 26 that would normally not be present on a two-wheeledleaning vehicle such as a conventional motorcycle. In this manner, theactuator 112 provides stability against tipping at low speeds, andallows the frame 58 to lean at higher speeds as freely as a motorcycle,but does not actively assist leaning. The map of K_(s) may optionally becalibrated to duplicate the ride feel of a particular motorcycle, forexample a sport motorcycle or a touring motorcycle, by experimentallydetermining the resistance of the particular motorcycle to leaning at arange of speeds and calibrating the map of K_(s) accordingly. In thisembodiment, the primary leaning torque for leaning the frame 58 of thevehicle 10 into a turn is generated by countersteering and by thegeometry of the vehicle 10.

Referring to FIG. 10D, according to a fourth embodiment K_(θ) is zero,and provides no restoring torque to limit or prevent leaning at lowspeeds. In this embodiment, the resistance to leaning at low speeds isprovided only by the geometry of the suspension of the vehicle 10, andis generally not sufficient to maintain the frame 58 in an uprightposition. In this embodiment, K_(s) is similar to the embodiment of FIG.10B, resulting in a vehicle 10 that is free-leaning at speeds below v₁and assists leaning at speeds above v₁.

In any of the embodiments described above, the control unit 200 mayadditionally use the signals it receives from the vehicle speed sensor202, the lateral acceleration sensor 204, and the lean angle sensor 208to determine whether an undesirable leaning condition is present orimminent. For example, a decrease in vehicle speed or a steering torqueapplied to the handlebar 20 by the driver may result in the currentlateral acceleration of the vehicle 10 being insufficient to maintainthe current lean angle, or may render the current lean angleuncomfortable for the riders. In this situation, the control unit 200may cause the actuator 112 to exert a restoring torque to move the frame58 toward an upright position or prevent an increase in the lean angle,irrespective of the values of K_(θ) and K_(s) determined from the maps.

It is contemplated that the control unit 200 may allow the driver of thevehicle 10 to select an operating mode from among a plurality ofoperating modes corresponding to the control maps in FIGS. 10A-10D. Inthis manner, the driver can select the desired degree of lean assistanceand the overall driving experience to be provided by the control unit200.

Modifications and improvements to the above-described embodiments of thepresent invention may become apparent to those skilled in the art. Theforegoing description is intended to be exemplary rather than limiting.The scope of the present invention is therefore intended to be limitedsolely by the scope of the appended claims.

1. A method of operating a leaning vehicle, the vehicle comprising: aframe having a front portion and a rear portion; a pivotable framemember pivotally connected to the frame about a generally horizontalpivot axis, the frame being pivotable relative to the pivotable framemember between an upright position and a plurality of leaning positions;a steering assembly supported by the frame for steering the vehicle; andan actuator having a first portion connected to the frame and a secondportion connected to the pivotable frame member, the actuator beingoperative to selectively pivot the frame with respect to the pivotableframe member about the pivot axis; the method comprising: determining asteering torque exerted on the steering assembly; determining a speed oftravel of the vehicle; determining a leaning angle of the frame withrespect to a reference angle; and exerting a leaning torque on the framerelative to the pivotable frame member about the pivot axis in a firstdirection using the actuator in response to a steering torque exerted onthe steering assembly in a second direction opposite the first directionwhen the speed of travel is above a threshold speed, thereby causing thevehicle to turn in the first direction.
 2. The method of claim 1,further comprising preventing the frame from pivoting about the pivotaxis relative to the pivotable frame member when the speed of travel iszero.
 3. The method of claim 1, further comprising exerting a leaningtorque in the direction opposite the leaning angle using the actuatorwhen a reduction in the magnitude of at least one of the steering torqueand the speed of travel is detected.
 4. The method of claim 1, wherein:determining the leaning angle of the frame with respect to the referenceangle comprises determining the leaning angle of the frame with respectto a vertical orientation.
 5. The method of claim 1, wherein:determining the leaning angle of the frame with respect to the referenceangle comprises determining the leaning angle of the frame with respectto the pivotable frame member.
 6. The method of claim 1, wherein:determining the leaning angle of the frame with respect to the referenceangle comprises determining a leaning angle of the frame with respect toa road surface.
 7. The method of claim 1, wherein the threshold speed isa first threshold speed; and further comprising exerting the leaningtorque using the actuator in a direction opposite the leaning angle atleast when the speed of travel is below a second threshold speed, thesecond threshold speed being less than the first threshold speed.
 8. Themethod of claim 7, wherein: when the speed of travel is below the secondthreshold speed, the leaning torque exerted by the actuator increases inmagnitude as the speed of travel decreases.
 9. The method of claim 7,further comprising preventing the frame from pivoting about the pivotaxis relative to the pivotable frame member when the speed of travel iszero.
 10. The method of claim 7, wherein the leaning torque is in thedirection opposite the leaning angle when a reduction in the magnitudeof at least one of the steering torque and the speed of travel isdetected.
 11. The method of claim 7, wherein: determining the leaningangle of the frame with respect to the reference angle comprisesdetermining the leaning angle of the frame with respect to a verticalorientation.
 12. The method of claim 7, wherein: determining the leaningangle of the frame with respect to the reference angle comprisesdetermining the leaning angle of the frame with respect to the pivotableframe member.
 13. The method of claim 7, wherein: determining theleaning angle of the frame with respect to the reference angle comprisesdetermining a leaning angle of the frame with respect to a road surface.